Animando com Opacity

gistfile1.txt

return AnimatedOpacity(
      opacity: _isVisible ? 1.0 : 0.0,
      duration: Duration(milliseconds: 500),
      child: Align(
        alignment: Alignment.bottomCenter,
        child: Padding(
          padding: const EdgeInsets.only(bottom: 8.0),
          child: GestureDetector(
            child: Container(
              decoration: BoxDecoration(
                  borderRadius: BorderRadius.circular(50),
                  gradient: orangeGradiente()),
              width: MediaQuery.of(context).size.width - 100,
              height: 58,
              child: Center(
                  child: Text(
                "Emergência",
                textAlign: TextAlign.center,
                style: TextStyle(color: Colors.white),
              )),
            ),
            onTap: () {},
          ),
        ),
      ),
    );